1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell, especially to the improvement of the negative electrode collector thereof.
2. Prior Art
With the increased use of portable equipment such as video camera, radio cassette recorder and others, instead of expendable primary cells, the demand for rechargeable secondary cell is growing in the market.
Most of the secondary cells, available nowadays, are nickel-cadmium cells with alkaline electrolyte. However, on this kind of cells, there is a difficulty for increasing energy density as it's voltage is limited to about 1.2 volt. Also, this type of cell has a defect of 20% higher self-discharge rate under normal monthly temperatures.
On the other hand, there have been some proposals to use a non-aqueous solvent as electrolyte solution and to use light metal such as lithium for negative electrode. Such a non-aqueous electrolyte secondary cell, because of it's ultimate 3V or higher voltage, not only has a high energy density, but also maintains a low self-discharge rate.
However, even in such non-aqueous electrolyte secondary cells, there is a problem of short circuiting internally within the cell, which is caused by dendrite-like crystallizations of metal lithium in the negative electrode and is caused by the growth of the crystals until they finally form a contact with the positive electrode. This shortens cell life and is an obstacle for commercial use of these secondary cells.
For these reasons, the amalgamation of lithium with other metals in negative electrodes for non-aqueous electrolyte secondary cells has been considered. However, the alloys used for the negative electrodes are broken down into fine particles by repeated charging and discharging of the cells, which shortens cell life.
For this reason, a non-aqueous electrolyte secondary cell is disclosed in Japanese patent application laid-open publication No.62-90863, which provides a non-aqueous electrolyte secondary cell with carbonaceous materials such as coke and others as negative electrode active anode material. In this type of non-aqueous electrolyte secondary cell, doping and dedoping of lithium ions to/from the boundary area of the carbon layers or to/from the micropores of carbonaceous material is applied for cell reactions to eliminate the problem of lithium dendrite like crystal growth and atomization of negative electrode and to secure long cell life. As disclosed in Japanese patent application laid-open publication No.63-135099, when Li.sub.x MO.sub.2 (M denotes a transition metal or metals, x denotes 0.05.ltoreq.x.ltoreq.1.10) is applied, especially good secondary cells are provided having extended use lives and higher energy densities.
However, in these non-aqueous electrolyte secondary cells, wherein a carbonaceous material is employed as a negative electrode active anode material, as compared to using metal lithium as a negative electrode active anode material, in spite of their better cycle life and safety, they are somewhat inferior in terms of their energy density.
One reason suggested for the observed inferiority in energy density is the use of a binder for binding the powdery carbonaceous material.
More particularly, in order to form an electrode from a powdery carbonaceous material, the addition of binder is required, at a ratio of about 10 to about 20% by weight for binding the powder to itself and to a collector. Although the organic binders are needed to obtain useful cycle life and safety, the binders do not directly contribute to cell charging and discharging. Accordingly, the addition of binder has a negative effect on cell capacity, proportional to the amount of binder used.
One method for solving such a problem is disclosed in Japanese patent application laid-open publications Nos. 6-150,908 and 7-288,126 and others. Instead of using binders, solid organic materials or pitch are used which, after being processed by heating, are carbonized to provide an active anode material retaining agent which participates in the reversible charging and discharging reactions of active anode material.
In accordance with these publications, a negative electrode is prepared from a mixture of a carbonaceous material, a solid organic material or pitch and the like and a solvent, which is painted on both sides of a metal collector and heated. During this heating process, the solid organic material or pitch is carbonized and sintered. The carbonized solid organic material functions to bind negative electrode carbonaceous materials and also has the ability to dope and de-dope lithium during charging and discharging to contribute to enhanced cell capacity.
The carbonized solid organic material is present three-dimensionally in the spaces between the particles of the carbonaceous material for the negative electrode. For this reason, in these sintered composite bodies, the binding agent does not represent a capacity loss, but instead serves to provide a durable, high energy density active anode for the secondary cell.